Redox stimulated variable-modulus material

ABSTRACT

A material having a first non-zero elastic modulus capable of reversibly changing the first non-zero elastic modulus to a second non-zero elastic modulus in response to a redox reaction occurring in the material. A method of producing a material that is reversibly cyclable between a first non-zero elastic modulus and a second non-zero elastic modulus, comprising: preparing a polymer comprising both crosslinks that do not depend on metal binding and functional groups capable of having oxidation-state specific binding constants to a metal ion; and doping the polymer with a solution containing the metal ion.

RELATED APPLICATION

The present application is a DIVISIONAL of copending U.S. patentapplication Ser. No. 13/710,114 entitled “REDOX STIMULATEDVARIABLE-MODULUS MATERIAL” and filed Dec. 10, 2012, the entirety ofwhich is incorporated herein by reference for all purposes. Thisapplication claims priority benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 61/568,835, filed Dec. 9, 2011.

GOVERNMENTAL RIGHTS

This invention was made with government support under Grant No.CHE0906980 awarded by the National Science Foundation (NSF). Thegovernment has certain rights in the invention.

BACKGROUND

A need exists for materials whose properties can be adjusted on-demandwithout requiring a change in the overall environment of the material.In the domain of “smart materials,” a wide variety of materials existthat exhibit response to stimulus, including piezoelectric (whichproduce electrical field and charge in response to applied stress orconversely undergo strain in response to applied electric field),magnetostrictive and electrostrictive (which exhibit strain in responseto magnetic or electric field), electroactive polymers (which exhibitstrain/swelling in response to application of electric charge), shapememory polymers and alloys (which exhibit strain in response to thermalor magnetic stimulus) and light-activated shape memory polymers (whichexhibit strain in response to light stimulus). In each of thesematerials, the primary effect is change in dimension and observablechange in modulus is a secondary (and minor) effect.

Material properties can be dramatically changed with chemical inputs,including pH, counterion identity and concentration, and chemospecifichost/guest interactions. Responses observed include phase transitions,changes in coordination or hydrogen bonding, electrostaticrepulsions/attractions, swelling/deswelling and conformational changes.A wide variety of photo-crosslinking materials are currently availableand are used in applications such as photoresists for microfabrication.While these materials do exhibit substantial change in mechanicalproperties, the reactions are not generally reversible and/or requirethe addition/removal of chemical reagents. Reversability is a desiredproperty. The addition/removal of chemical agents is an undesirablerequirement.

Electro- and magneto-rheological (ER and MR) fluids are known for theirreversible changes in viscosity due to applied electrical or magneticfield, however the effects disappear when the stimulus is removed, andthey do not affect the elastic properties of the material.Magnetorheological elastomers are a derivative of MR fluids in which themagnetic materials are bound in an elastomeric medium so thatapplication of magnetic field changes the elastic properties of thecomposite structure. The stimulus is magnetic (not electrical) and itdoes not exhibit a power-off hold state. That is, when the magneticfield is removed, the material reverts to its base (soft) state.

Polyelectrolyte-based hydrogels (electroactive polymers, EAPs), have theability to behave as artificial muscles which bend directionally when apotential gradient is applied. The “bending” behavior is driven by ionmigration and osmotic pressure and can, therefore, occur only during theactual application of electrical energy.

Another desired property is the maintenance of a three-dimensional shapein all states. Electrically-stimulated polymeric materials that exhibitmechanical property changes other than osmotically-controlled mechanicalactuation are generally stimulated either as cast films (not macroscopicin all dimensions), or they undergo a transformation between sol and gelstates (shape is neither controlled nor maintained).

Forming and breaking polymer chain crosslinks can change bulk mechanicalproperties. However, few of these materials are reversible and of thosethat are, all have stimulus-defined limitations. For example, manysystems are not self-contained—they require manual addition and removalof solvents or chemicals for each response. Other systems are stimulatedby temperature which is difficult to direct to a specific location inthe material. Moreover, the required activation temperatures could proveimpractical to access and/or implement for specific applications.

The fundamental redox properties and complexation differences of ironand copper in multiple oxidation states have been reported to introducecrosslinks into linear polymers. These systems are soluble liquids inone oxidation state and dimensionally undefined gels formed by kineticprecipitation in the other. Example systems have demonstrated thateither electrochemistry or light can be used in the Fe²⁺/Fe³⁺ redoxcouple to induce a sol-gel transition in poly(acrylic acid).

Polyelectrolytes systems can be chemically or electrochemically switchedbetween two states by exploiting redox sensitive couples suchferrocene/ferrocenyl and Fe(CN)₆ ⁴⁻/Fe(CN)₆ ³⁻. The observedswelling/deswelling and aggregation/deaggregation behaviors of thesesystems originate from the differences in intra- and interchainelectrostatic interactions caused by the change in overall charge on themetal complexes rather than by changes of coordination at the metalcenter.

While modulus change is inherent in the materials listed above, it isnot the primary feature of many of the materials. For example, the shapememory materials have an inherent transition temperature (or magneticfield), which if exceeded, the material will rearrange its structure,thereby undergoing substantial strain (up to 8% for alloys and up to100% for polymers). There is an accompanying change in elastic modulus(which depends on loading conditions and may be 2-3 times for alloys andorders of magnitude for polymers). The fact that the modulus changecannot be controlled independently of the dimensional change makes itimpractical as a useful feature, except in very limited applications.The modulus change has been used more extensively in shape memorypolymers than in alloys, partly because it is more pronounced in thepolymers, but more importantly because the stresses that can besupported by the polymers are much lower than in the alloys, so ifconstrained the modulus change is the more dominant effect. For both ofthese systems, though, the more limiting factor is the thermal stimulus,which is difficult to control spatially, results in a very slow responsetime for material transition, and has no ability to hold the materialsin the soft modulus with no power (note that for magnetic shape memoryalloys, the response time is much faster, but the lack of power-off holdis still problematic as is the geometric issue of applying thestimulus).

SUMMARY

A first preferred aspect of the present application is a material havinga first non-zero elastic modulus capable of reversibly changing thefirst non-zero elastic modulus to a second non-zero elastic modulus inresponse to a redox reaction occurring in the material. In a preferredembodiment the redox reaction is caused by an electric potential appliedacross the material or by exposing the material to an oxidant orreductant. In a preferred embodiment the first non-zero elastic modulusor second non-zero elastic modulus is maintained by the material afterabatement of the redox reaction. In a preferred embodiment the material,in-whole or in-part, may reversibly change from substantially the firstnon-zero elastic modulus to substantially the second non-zero elasticmodulus and vice versa upon successive redox reactions occurring in thematerial, in-whole or in-part. In a preferred embodiment the materialhas a minimum elastic modulus and a maximum elastic modulus, whereineach of the first non-zero elastic modulus and the second non-zeroelastic modulus may consist of a value at the minimum elastic modulus,the maximum elastic modulus or a value therebetween wherein the firstnon-zero elastic modulus is greater than the second non-zero elasticmodulus or the first non-zero elastic modulus is less than the secondnon-zero elastic modulus. In a preferred embodiment the material definesa first shape while the material has the first non-zero elastic moduluswhich is less than the second non-zero elastic modulus, wherein thefirst shape may be deformed to define a second shape and wherein thesecond shape is maintained after application to the material of a redoxreaction and wherein the material attains the second non-zero elasticmodulus. In a preferred embodiment the material is a component of acomposite structure.

A second preferred aspect of the present application is a materialreversibly cyclable between a first non-zero elastic modulus and asecond non-zero elastic modulus comprising one or more metal ions and apolymer having both crosslinks that do not depend on metal binding andfunctional groups that have oxidation-state specific binding constantsto the metal ions. In a preferred embodiment the metal ions consist ofonly one element. In a preferred embodiment the metal ions consist of atleast two different elements and the polymer has both crosslinks that donot depend on metal binding and functional groups that haveoxidation-state specific binding constants to each element. In apreferred embodiment the polymer contains one or more functional groupsselected from the group consisting of: carboxylate, carboxylic acid,amide, sulfide, thiol, alkoxide, alcohol, phosphine, amine, amide,halogen, sulfonic acid, sulfonate, phosphonate and phosphonic acid;wherein the one or more functional groups bind to the metal ions. In apreferred embodiment the one or more metal ions are selected from thegroup consisting of: iron (II/III), copper (I/II), cobalt (II/III),palladium (0/II), silver (I/II), manganese (II,III,IV), nickel (0,II),ruthenium (II/III), rhodium (I/II/III), platinum (MI) and gold (0/I/II).In a preferred embodiment the polymer comprises monomer residues frommonomers having the structure H₂C═CR¹R², where R¹ and R² areindependently selected from the group consisting of —H, (C1-C8)alkyl,phenyl, aryl, heteroaryl, pyridinyl, pyrrolyl, thiophenyl, —C(═O)OR³,—C(═O)NR³R⁴, —C(═NR³)OR⁴ and —CH═CR³R⁴, where R³ and R⁴ areindependently selected from the group consisting of —H, (C1-C8)alkyl,phenyl, aryl, heteroaryl, pyridinyl, pyrrolyl and thiophenyl. In apreferred embodiment the polymer is a hydrogel and comprises at leastone monomer residue selected from the group consisting of:4-vinylpyridine, acrylate, styrene sulfonate and polyethylene glycoldiacrylate. In a preferred embodiment the one or more metal ions isselected from the group consisting of copper (I/II) and iron (II/III).In a preferred embodiment the polymer further comprises a least oneconductive additive selected from the group consisting of: carbonnanotubes, chemically modified carbon nanotubes, graphene,partially-reduced graphene oxide, chemically modified graphene,conducting polymers and nanoparticles.

A third preferred aspect of the present application is a method ofproducing a material that is reversibly cyclable between a firstnon-zero elastic modulus and a second non-zero elastic modulus,comprising: preparing a polymer comprising both crosslinks that do notdepend on metal binding and functional groups capable of havingoxidation-state specific binding constants to a metal ion; and dopingthe polymer with a solution containing the metal ion. In a preferredembodiment the method further comprises reversibly cycling between afirst non-zero elastic modulus and a second non-zero elastic modulus byaltering the oxidation state of the metal ion by a redox reaction or aredox stimulus. In a preferred embodiment the solution further comprisesat least one ligand with functional groups capable of binding to themetal ion, wherein the functional groups are selected from the groupconsisting of: carboxylate, carboxylic acid, amide, sulfide, thiol,alkoxide, alcohol, phosphine, amine, amide, halogen, sulfonic acid,sulfonate, phosphonate and. In a preferred embodiment the solutioncomprises a copper salt and urea. In a preferred embodiment the solutioncomprises an iron salt and citric acid. In a preferred embodiment theiron salt is FeCl₂ or FeCl₃. In a preferred embodiment the polymer is across-linked hydrogel made by a first step of polymerizing an aqueoussolution comprising sodium acrylate, sodium sulfonate, and polyethyleneglycol diacrylate with ammonium persulfate. In a preferred embodimentthe redox stimulus is delivered by application of an electric potentialacross the material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Electroplastic elastomer mechanism. Multi-step pathway thatreversibly converts electricity to a change in bulk stiffness iniron-crosslinked electroplastic elastomer hydrogels.

FIG. 2. Redox-mediated switching between second non-zero elastic modulusand first non-zero elastic modulus states for iron-based electroplasticelastomer. Reversible electrochemical conversion of second non-zeroelastic modulus Fe³⁺-crosslinked hydrogel (left) to first non-zeroelastic modulus Fe²⁺ hydrogel (right). Hydrogel in oxidized (left) andreduced (right) states held in gloved hand. (Bottom) Mechanicalstress/strain curves for EPEHs in the oxidized and reduced states undercompressions.

FIG. 3: A Cu²⁺-coordinated sample is stiff, whereas the colorlessCu⁺-coordinated sample is soft and bends in response to gravity.

DETAILED DESCRIPTION

It is to be understood that the descriptions of the present disclosurehave been simplified to illustrate elements that are relevant for aclear understanding of the present disclosure, while eliminating, forpurposes of clarity, other elements that may be well known. Those ofordinary skill in the art will recognize that other elements aredesirable and/or required in order to implement the present disclosure.However, because such elements are well known in the art, and becausethey do not facilitate a better understanding of the present disclosure,a discussion of such elements is not provided herein. Additionally, itis to be understood that the present disclosure is not limited to theembodiments described above, but encompasses any and all embodimentswithin the scope of the description and the following claims

DEFINITIONS

The following definitions apply to the terms as used throughout thisspecification, unless otherwise limited in specific instances.

As used herein, the term “modulus” is defined as the slope of itsstress-strain curve in the elastic deformation region. As such, astiffer material will have a higher elastic modulus.

As used herein, the term “redox” is defined as reduction-oxidationreactions and include all chemical reactions in which atoms have theiroxidation state changed

As used herein, the term “oxidation” is the loss of electrons or anincrease in oxidation state by a molecule, atom, or ion.

As used herein, the term “reduction” is the gain of electrons or adecrease in oxidation state by a molecule, atom, or ion.

As used herein, the teim “metal ion” includes metals atoms in the zero,+1, +2, +3, or +4 oxidation state.

As used herein, the term “permanent” when used to describe polymericcross-links means cross-links of a covalent nature that are not readilycleavable, i.e., cross-links formed by one or a series of covalentbonds, such as covalent single bonds, covalent double bonds and covalenttriple bonds

DESCRIPTION

The present specification describes the creation of a new material thatuses electricity or a change in oxidation state as a stimulus toproduce, reversibly, a change in bulk-scale stiffness as a response (seeFIG. 1). This novel class of materials is known as electroplasticelastomer hydrogels (hereinafter “EPEHs”). EPEHs represent the first ofa novel class of materials that act in a self-contained system to changemechanical properties with electrical stimulus or change in oxidationstate. The EPEH materials are created with defined dimensions and whilethere may be a change in volume due to differences in water swelling,the dimensions remain well-defined rather than going through a liquidstate. The availability of materials of this type spawn new designparadigms that in turn lead to innovations in aerospace, manufacturing,consumer products, robotics, etc.

Although the creation of materials that respond to external stimuli isone of the most active frontiers of current materials development, EPEHsdisplay a unique and valuable combination of properties not found in anyother system: 1) reversible changes in mechanical stiffness which can beachieved using either electrical input or change in oxidation state and2) 3D-macroscale dimensions in all states. Electricity, which isemployed as one of the stimuli for EPEHs, satisfies these requirementsand offers practical advantages including ease of access, portability,and a sophisticated technology infrastructure.

EPEHs maintain a three-dimensional shape in all states, which is aproperty not shared by other electrically reversible systems. EPEHs arenot limited to, but can have macroscopic dimensions in all directionsand maintain a non-zero stiffness in all states that enables shape to beretained while compliance is tuned.

The Fe²⁺/Fe³⁺ redox couple and the Cu⁺/Cu²⁺ redox couple were utilizedfor developing the EPEHs of the present specification because thesesystems are both well-behaved and well-understood; these ion pairs canbe interconverted in a convenient electrical potential window. As ironor copper ions in different oxidation states have distinct coordinationpreferences—Fe³⁺ binds more strongly than Fe²⁺ to “hard” ligands—thechange in oxidation state can be used to control the degree ofcrosslinking in a polymer bearing hard carboxylate side-groups. Giventhe known correlation between crosslink density and the stiffness ofpolymeric materials, it follows that the mechanical properties of thebulk material should be reversibly controlled by the interconversion ofFe²⁺ and Fe³⁺. or Cu⁺ and Cu²⁺

Samples prepared independently with comparable Fe²⁺ and Fe³⁺ ioncontents (Table 1, ca. 1.2 mmol/cm³) exhibited more than an order ofmagnitude difference in modulus when subjected to mechanical testingusing an indentation methodology. Compressive moduli of 0.06 and 2.1 MPawere measured for Fe²⁺ and Fe³⁺ samples, respectively, that wereprepared, measured, and analyzed for iron content using identicalprotocols. Moduli higher than 2.1 MPa can be achieved for Fe³⁺ samplesby adjustments in doping conditions.

The mechanical properties of the EPEH samples are controlled by theelectrolytic interconversion of the Fe³⁺ and Fe within the same bulksample. An EPEH sample of standard dimensions was prepared directly on aglassy carbon electrode. After in-situ Fe³⁺ exchange the sample wasprotected from exposure to light and subjected to a reducing potentialof −0.8 V for 18 hours in an electrolyte solution of 0.5 M citric acidand 2.0 M FeCl₂. The sample became softer to the touch, paleorange-yellow in color, and was visibly swollen relative to the initialstate (FIG. 2-top right). Exchange of the tightly bound Fe³⁺ with theFe²⁺ present in the electrolyte solution (necessary for the reductionstep in samples that will be cycled between states, vide infra) is notsignificant—a control submerged for the same period in the same solutionwithout electrolysis, did not soften nor change color. It is importantto note that the reduction occurs analogously when the electrolytesolution comprises only KNO₃ (0.2 M, pH 1). Also, leaching ofhydrogel-bound Fe³⁺ into the electrolyte solution is negligible underthese conditions. Mössbauer analysis of both the starting sample and thesample produced by reduction established unambiguously that a nearlycomplete conversion of the high-spin Fe³⁺ in the sample to high-spinFe²⁺ occurred. Air oxidation during Mössbauer sample preparation and/orincomplete reduction is responsible for the small Fe³⁺ shoulder (<15%).The sample color for the reduced EPEH, which is orange-yellow ratherthan the yellow-green that is characteristic of freshly preparedFe²⁺-doped hydrogels, is likewise consistent with the presence of asmall fraction of the more intensely colored Fe³⁺ crosslinks

Oxidation of a freshly prepared Fe²⁺ EPEH in 2 M FeCl₂, 0.5 M citricacid produced the opposite changes in color and mechanical properties.After oxidation at 1.2 V for a period of 14 hours (light excluded, N₂atmosphere), the sample became darker orange in color, thinner, andstiffer (FIG. 2—top left); grid pattern caused by macroporous pressurecap). The presence of FeCl₂ in the electrolyte facilitates the oxidationstep because, as per the design of the system, Fe²⁺ is weakly bound andwill, therefore, rapidly equilibrate with the external solution. (FIG.2—bottom) shows stress strain curves that were acquired by indentationtesting of electrode-mounted samples after oxidation (left) andreduction (right). Chemical oxidation of Fe²⁺ samples by treatment withammonium persulfate gave analogous physical and optical changes. EPEHswith Fe²⁺ crosslinks also slowly oxidize in air over the course of hoursto days, as shown by changes in color and stiffness of samples stored inhumid environments to prevent drying.

The oxidation/reduction is reversible. The compressive moduli for asingle EPEH sample that was subjected to two cycles of reduction andoxidation switch reversibly between ca. 1.0 MPa and 0.6 MPa. At eachstage the samples displayed the color profile and degree of swellingthat is characteristic of the particular oxidation state. Although thechanges are reproducible and the moduli are clearly distinct, thedifference in modulus range is smaller than that observed for samplesdirectly prepared from Fe²⁺ and Fe³⁺. While not intending to be limitedby any theory, we attribute the differences to a combination of twofactors: 1) iron equilibration between the sample and electrolyte underexperimental conditions and 2) air oxidation of reduced samples duringsample transport and mechanical measurement.

Chronoamperometry and chronocoulometry establish that the redox processfor sample of dimensions 2.5×2.5×o.2 cm requires >14 h. It should benoted that the total charge passed is much greater for the oxidationprocess because of the presence in the electrolyte solution of excessFe²⁺, which is maintained in constant excess within the system—not addedor removed—for both the oxidation and reduction cycles.

EPEHs manifest a combination of features that suggest that they have anexceptional potential for further development and applications:scalability, reversibility, stability, tunability, and effectivedelivery of the stimulus.

Scalability is a key characteristic of the EPEH materials. Manyintriguing nano- and subnanoscale phenomena have not successfully beentranslated into macroscale responses. By employing Nature's tactic ofusing multiple mediating steps it has been possible to translate anatomic scale phenomenon, metal-ion redox transformation, to a mechanicalresponse that is readily observable on a macroscale. The hydrogels areprepared from non-exotic reagents and the same basic procedure isapplicable to samples on larger scales—we have prepared samples withthicknesses up to 2.5 cm and length×width dimensions>100 cm².

Reversibility and stability of the different states are features of theEPEHs. The redox process cycles the metals between two states that arestable as long as the material is protected from environmental oxidantsand reductants. The electrical power used to switch states is notnecessary to maintain them. There is also no theoretical limit on thenumber of times that the electrochemical process can be repeated. Anaqueous Fe²⁺ reservoir is a component for the cycling as the uptake andexclusion of water and ions in the hydrogel is integral to themanifestation of oxidation-state dependent mechanical properties.

EPEHs are highly tunable both in their preparation and in theirimplementation. By varying the percentage of carboxylate monomers orPEG-DA crosslinking agent relative to the other components, thefundamental stiffness can be adjusted within the limits of maintainingsample integrity and hindering ion migration. There is also thepotential to adjust the stiffness through a full continuum of valueswithin its range by partial redox.

EPEH samples prepared with the addition of 1-3% vinyl-functionalizedmulti-walled carbon nanotubes (MWNTs) or graphene oxide were doped withiron and then subjected to reducing conditions. The time to pass 40Coulombs decreased from 11.9 h for hydrogel with no nanotubes to 3.2 hfor 3%-MWNTs. Qualitative examination of the hydrogel color and behavioris consistent with a significant decrease in time for iron reduction.

Use of Carbon Nanotubes for Improving Conductivity and Response Time:

To improve response time for electrochemical redox conductive elements,for example, carbon nanotubes, chemically modified carbon nanotubes,graphene, partially-reduced graphene oxide, chemically modifiedgraphene, conducting polymers and metal nanoparticles, can be added tothe hydrogel. These conductive elements will decrease the diffusiondistance for the metal and reinforce the material.

Use of Copper as the Binding Metal:

The mechanical properties of EPEHs, and therefore, their potential forapplication depends on their exact formulation. A new electroplasticelastomer hydrogel has now been prepared that uses the differentcoordinative abilities of the Cu⁺/Cu²⁺ pair to control materialstiffness; Fe²⁺/Fe³⁺ was used previously. The polyelectrolyte backboneof the new hydrogel bears new ligands which are more suited to copperbinding, in place of the iron-binding ligands used in the firstformulation. After doping with Cu²⁺, the dark-blue hydrogels, which aremuch stiffer and tougher than the Fe³⁺ gels, can be reducedelectrochemically to give a light-green Cu⁺ hydrogel that is veryflexible. These new copper-based hydrogels will have application where atougher, more elastic interface is needed.

Prosthetics and Orthotics Applications:

Potential applications for variable modulus materials are widespread andinclude such areas as robotics, aerospace structures, adaptive opticsand medical devices. The range of moduli and the ability to tune themodulus through a continuum of values as a function of stimulus makethem suitable for potential uses in orthotics and prosthetics in whichthey serve as customizable interfaces between the relatively softexterior of the human body and more rigid protective or prosthetic gear.These materials provide an ability to customize fit and to adjust thestiffness of the material as often as needed to increase comfort and/orimprove function.

Although there have been significant technological leaps in the designand function of prosthetics, the interface with the human body remains aweak point. EPEHs may allow the user to control and customize the fit torespond to changes in their bodies (e.g. swelling, weight loss),environment (e.g. temperature, humidity) or task (e.g. normal dailyactivity vs running a 10 K race). Another specific medical applicationis orthotics for the prevention of pressure ulcers. For example,diabetic foot ulcers alone make up a large medical problem, withestimates of up to 25% of the estimated 20 million people with diabetesin the US expected to develop a diabetic foot ulcer within theirlifetimes. The EPEH material in this invention may be used to developorthotics which are locally tunable for adaptive treatment of suchulcers. Pressure ulcers also originate in many other instances such asthe use of prosthetics and wheel-chairs, and bed-ridden patients.

The electrically-activated materials lead to flexible, tunable materialssuitable for application in the prevention of ulcers in the diabeticfoot. Diabetes is already a major health concern, and still rapidlygrowing, that in many cases exhibits a well understood path ofneuropathy in the foot, to pressure ulcers, to amputation, andultimately to premature death. Foot ulcers may be prevented byelectrically tuning an insole device's stiffness to reduce high stressareas thereby preventing the onset of life-threatening conditions thatoften evolve from diabetic foot ulcers.

The hydrogel material maintains its structural integrity during thetransition but deforms easily under stress such that it can serve as a“cushion” for example between the relatively soft tissue of the humanbody and harder surfaces. The working range of moduli is tunable bychanging the chemical composition of the material and lies in the kPa toGPa region, which is ideal for this class of application. Because themodulus change can be addressed spatially using independent EPEH cells,an adjustable interface between a foot and an insole, for example, couldbe produced, with softer areas seeing reduced stress and harder areascarrying more stress. The synthetic process is facile and can yieldsamples of virtually any dimensions. The EPEH system involves theelectro-adaptive hydrogel, electrodes, and an electrolyte solution thatprovides a reservoir of ions that enable the red-ox reactions. Thesecomponents may be contained in a set-up conducive to laboratory benchtesting.

Array and/or Multi-Layer Cells:

Arrays and/or layers of multiple EPEH cells or units may be used toachieve spatially addressable structures of sufficient thickness toaccommodate the expected deformations, but thin enough to achieve theneeded electron and ion transport. EPEH units may be in many differentshapes including cylindrical, hollow tube, prismatic, strand, layer, andmultiple units may be combined in two or three-dimensional arrays, rows,columns, layered, woven, or braided arrangements to create an article.Alternatively, articles can be made of a single quantity of EPEHmaterial in many different shapes.

Development of Cell “Skin”:

A skin material may be developed that serves the dual purpose ofcontainment of the electrolyte reservoir and interface to an adjacentsurface. The skin enables the EPEH material to maintain a pliableinterface to an adjacent surface in the face of thermal, moisture, andshear stress issues.

The most important elements of diabetic foot ulcer (DFU) preventionstrategies are the appropriate use and design of footwear. Current bestpractices for prevention include the design of custom fit shoes withmolded inserts designed to minimize the harmful pressures that goundetected by the person with neuropathy in their feet. The highincidence and prevalence of DFUs provide clear evidence that thesestrategies are woefully inadequate. Part of the problem can beattributed to low compliance with footwear interventions. A study oncompliance to diabetic footwear interventions showed that only 42% ofthe people followed wore their shoes long enough to reach the dailyeffectiveness threshold of 60%. Indoors, where many people spend most oftheir time, the compliance was worse—only 30%.

However, the root cause of the problem is poor fitting footwear thatdoes not redistribute pressure well under changing conditions. The stateof the art customization procedures that are practiced today typicallyuse an interface pressure distribution measurement made under staticstanding conditions on a rigid flat surface to design a custom shapedorthotic supporting surface. The resulting orthotic does not and cannotaccount for changing load distributions with gait or changes resultingfrom swelling. Optimization of the load distribution is a daunting taskwith the currently available materials and knowledge. While it ispossible to measure interface pressure dynamically during gait, it isdifficult and impractical to iteratively alter support surface shapeand/or stiffness to provide an optimum solution.

An EPEH and control system embedded in either a sock or insole thatallows for rapid optimization by manipulating stiffness andredistributing pressure in the foot can be prepared. The smart orthoticfootpad may comprise an array of up to a few thousand electrode elementsand will have an overall thickness between 1 and 10 mm. The device thenwill comprise an array of independently addressable areas whosestiffness can be adjusted.

EXPERIMENTALS

Typical Hydrogel Preparation for Iron Articles:

EPEH samples were prepared by simple free-radical copolymerization ofcommercially purchased monomers under standard conditions. Sodiumacrylate, sodium (4-styrene sulfonate), and polyethylene glycoldiacrylate (PEG-DA, M_(n)=575) in a weight ratio of 12:8:1 were reactedin aqueous solution with an ammonium persulfate catalyst at 85° C. for1.5 hours to give a soft, colorless hydrogel. The presence of thepermanent PEG-DA crosslinks gives the hydrogels a baseline shape definedby the reaction vessel. Adjustments in PEG-DA stoichiometry relative tothe other monomers produced hydrogels that were qualitatively stiffer(increased PEG-DA) or softer (decreased PEG-DA).

Iron Doping:

Cation exchange of sodium ions for Fe²⁺ or Fe³⁺ was accomplished bysubmersion of the hydrogel in a solution of 2.0 M FeCl₂ or FeCl₃ and 0.5M citric acid for a period of 20-48 hours. Exchange with Fe²⁺ producedsamples that were pale yellow-green in color and slightly smaller thanthe original hydrogel, due to coordinative crosslinking. Samplesprepared with Fe³⁺ were orange-red and even more contracted indimension—up to 50% smaller in thickness than the pre-doped samples.Hydrogels were transparent and appeared homogeneous throughout. Althoughthe standard samples prepared for this article are relatively small,2.5×2.5×0.2 cm after doping with Fe³⁺, the procedure is inherentlyscalable to nearly any sample size.

Depending on the dimensions of the sample being prepared, 2 to 8 mL ofthe reaction mixture was pipetted into a mold. For electrochemicalexperiments the mold for the sample was created by temporarily affixing,using Poly(dimethylsiloxane) (PDMS) adhesive, a square glass cell to aTeflon base bearing a freshly polished glassy carbon electrode (GCE).The mold/sample combination was then heated at 85° C. for 1.5 h. Aftercooling to RT, the hydrogel was doped by simple submersion in either asolution of 2.0 M FeCl₂/0.5 M citric acid or 2.0 M FeCl₃/0.5 M citricacid for a period of 20-48 h. A 1:3 ratio by volume of doping solutionto pre-polymer was used.

Hydrogel Preparation Imbedded Conductors:

Vinyl-functionalized multi-walled carbon nanotubes (MWNTs) weresynthesized from COOH-functionalized multi-walled carbon nanotubes(COOH-MWNTs, diameter: 8-15 nm, length: 10-50 um, 2.56% (w/w functionalcontent)). Graphene oxide 915 mg/ml of water) was prepared by Hummer'soxidation of natural flake graphite. Prior to hydrogel polymerizationMWNTs or graphene oxide flakes were suspended in DI-water and dispersedin an ultrasonic water bath for 30 min The dispersed semi-conductorswere then added to the dissolved monomers (mixed in the same ratio asfor simple hydrogels) and APS was added as a radical initiator.Polymerization and iron doping was performed as described above.

Mössbauer Spectroscopy:

The ⁵⁷Fe Mössbauer spectra were collected on constant accelerationinstruments over the temperature range of 4.2-300 K in zero or 0.045 Tapplied fields. Samples were prepared by adding minced hydrogel (1-5 mm²pieces) to Teflon Mössbauer cups covered with lids. Spectral simulationswere generated using WMOSS (WEB Research, Edina, MN). Isomer shifts arereported relative to Fe metal foil at room temperature.

The room temperature Mössbauer spectrum of a sample of the Fe³⁺-dopedhydrogel showed one quadrupole doublet with an isomer shift of δ=0.41mm/s and a quadrupole splitting of ΔE_(Q)=0.53 mm/s These Mössbauerparameters confirm the presence in the hydrogel of high-spin Fe³⁺. Theyare also similar to Mössbauer parameters of high-spin Fe³⁺ ions inoxalates (δ between 0.35 mm/s and 0.41 mm/s and ΔE_(Q) between 0.38 mm/sand 0.75 mm/s)

The 4.2-K Mössbauer spectrum of a similar sample of the iron-dopedhydrogel that was electrochemically reduced to Fe²⁺ showed a quadrupoledoublet with δ=1.37 mm/s and ΔE_(Q)=3.26 mm/s, which represents 85% ofthe iron in the sample. These parameters are typical of high-spin Fe²⁺and are comparable, although at the high end, of the Mössbauerparameters of Fe²⁺ in oxalates. This result confirms the efficiency ofthe reduction protocol. A small shoulder on the right side of the leftline of the Fe²⁺ quadrupole doublet indicates the presence in the sampleof a small amount of high-spin Fe³⁺. Note: spectrum collected at lowtemperature to inhibit oxidation during data collection.

Mechanical Measurements:

The mechanical testing procedure, specifically developed for the case oftesting thin EPEH materials, was based on an indentation testingmethodology. A circular cylindrical indentation probe (diameter 6.2 mm)was fashioned to screw into the crosshead of an MTI-1K screw driven,table top load frame. A 10N Transducer Techniques load cell was employedto measure the force exerted on the EPEH specimen by the indentationprobe. Owing to the thin nature of the specimens tested (<10 mm), aswell as the small range of expected loading, the strain was calculatedfrom the crosshead displacement as opposed to using an externalextensometer. Additional experimental parameters such as strain rate andtotal strain were determined by referring to ASTM D1621-04A StandardTest Method for Compressive Properties of Rigid Cellular Plastics. Eachindentation test yielded a single stress-strain curve, which contributeda single stiffness measurement (Young's modulus). In total, fiveindentation tests were performed on each 2.5×2.5×0.2 cm sample (one ineach corner, and one in the center of the sample) and the mean value wasreported. Per the standard, Young's modulus is measured by taking theslope of the linear portion of the curve.

Electrochemical Measurements.

Cyclic voltammetry (CV) and amperometry measurements were carried outwith a CH Instruments Electrochemical work station Model 430A (Austin,Tex.) at RT using a three-electrode system composed of a glassy carbonplate (GCE, 25×25 mm) working electrode, a Ag/AgCl reference electrode,and a platinum grid counter electrode. The GCE was polished with 0.3 μmAl₂O₃ paste and cleaned thoroughly in an ultrasonic water bath for 5 minprior to each use. The CV and amperometry experiments for reduction andoxidation were carried out in 15 mL of 2.0 M FeCl₂/0.25 M citric acid,pH ˜1.8. CV data were acquired at a scan rate of 100 mV/s over a voltagerange of 1.2 to −0.8 V. Bulk electrolysis was performed in the sameelectrolyte solution for up to 40 h (reduction potential −0.8 V,oxidation potential +1.2 V). All electrochemical experiments wereperformed under an N₂ atmosphere with careful exclusion of ambient lightto prevent the photoreduction of Fe³⁺ ions in the presence of citricacid.

CONTROL EXPERIMENTS

Bulk electrochemical reduction at −0.8 V of Fe³⁺-hydrogel in 15 mL ofKNO₃ (0.2 M, adjusted to pH 1) electrolyte was perfoimed for 16 h.Sample exhibited properties analogous to reductions performed understandard conditions (15 mL of 2 M FeCl₂/0.25 M citric acid, pH ˜1.8,16-20 h).

Fe³⁺-hydrogel samples showed negligible leaching of Fe³⁺ when soaked in15 mL of KNO₃ (0.2 M, adjusted to pH 1) over similar time periodswithout applied reduction potential. Fe²⁺-samples showed dramaticleaching into the electrolyte under similar conditions.

Fe³⁺-hydrogel samples showed negligible exchange when soaked in 15 mL of2 M FeCl₂/0.25 M citric acid, pH ˜1.8. The material retained both colorand stiffness over periods>20 h.

A Fe²⁺-doped sample was treated with 2M APS by a combination ofsubmersion (<1 hour) and intra-gel injection. The sample rapidly becamedark-orange in color, smaller in dimension and qualitatively stiffer.

A Fe²⁺-doped sample was exposed to atmospheric conditions in a closedcontainer under moisture conditions (reservoir of free water, coveredwith damp towel) known to prevent sample dehydration. The sample becameprogressively orange in color and stiffer over a period of hours.Consistent with increased Fe³⁺ crosslinking, some water loss from thegel occurs during this period, as indicated by sample shrinkage.

Chronoamperometry and Chronocoulometry for Redox Cycling of Fe³⁺Hydrogel

The sample used for the redox cycling was initially doped for 47 h toyield an ˜2 mm thick Fe³⁺ hydrogel. Redox cycles following the firstreduction were carried out for 15-18 h.

Quantification of Iron:

Iron-doped hydrogels were digested for 2 h using concentrated HCl (5 mLHCl per 1 mL pre-polymer volume). Two 100 μL aliquots from theHCl-degraded hydrogel were diluted in parallel in sodium acetate buffer(0.1 M, pH=4) so that the absorbance was in the linear range of theinstrument (10 mL final volume, denoted Samples A and B). To determinethe Fe²⁺ content, a solution of 1,10-phenanthroline in water (2 mL,0.0055 M) was added to Sample A and the absorbance was measured. Todetermine the total Fe content, Sample B was treated with an excess ofthe chemical reductant hydroxylamine.HCl (1.5 mL, 1.4 M in water). Afterreacting for 10 min a solution of 1,10-phenanthroline (2 mL, 0.0055 M)was added and the absorbance was measured. Fe³⁺ was determined bydifference.

Mechanical Properties:

Mechanical measurements and quantitative analysis were carried out onFe²⁺- and Fe³⁺-doped EPEHs (ca. 2.5×2.5×0.3 mm after doping). Theresults are summarized in Table 1. The FeCl₂- and FeCl₃-doped hydrogelscontained approximately the same amounts of total iron. When theparallel samples were mechanically tested an ˜36-fold difference wasobserved between their moduli. The iron to carboxylate ratio wascalculated assuming complete SA copolymerization (4 mL pre-polymer). Itis important to note that the only difference between these samples isthe oxidation state of the iron.

TABLE 1 Mechanical properties of Fe²⁺- and Fe³⁺-doped hydrogels.^(a)Fe²⁺ Fe³⁺ Young's Modulus Dopant (mmoles) (mmoles) (MPa) Fe:carboxylateFeCl₂ 2.116 — 0.06 1:2.6 FeCl₃ — 2.210^(b) 2.1 1:2.5 ^(a)Sample size ca.2.5 × 2.5 × 0.3 mm = 1.875 cm³; ^(b)Fe³⁺ per volume of 1.2 mmol/cm³.

Cupric Doped Hydrogels and Cuprous Doped Hydrogels:

Hydrogels were prepared in a one-pot reaction to be 40% ligand(4-vinylpyridine) by mass. These samples were clear, light yellow incolor, and pliable but somewhat brittle and easily cracked. Thesehydrogels were then doped with either copper(II) or copper(I). Dopingwith Copper (II) was accomplished by diffusion. The original hydrogelswere soaked in a doping solution overnight containing both CuCl₂ andurea, a moderate chelating agent. To optimize the concentration of CuCl₂and urea, an array of samples doped with different concentrations ofboth CuCl₂ and urea were tested. Variation of urea concentration in thedoping solution had no noticeable effect on the mechanical properties ofthe samples. However, variation of the copper concentration had aprofound effect on the doped samples. Samples doped with 2.0 M CuCl₂were deep green, soft, and pliable. Samples doped with 1.0 M CuCl₂ wereturquoise, somewhat harder than the 2.0 M samples, yet still pliable.Samples doped with 0.5 M CuCl₂ were bright sky blue, glassy, and hard.Samples also shrank considerably in size after doping. Samples weretough and robust, but could be bent if enough force was applied.

Larger samples doped with 0.5 M CuCl₂ and 0.025 M urea were prepared.These larger samples were suitable for tensile testing, and were testedfor mechanical strength. The average Young's Modulus of these materialsin the second non-zero elastic modulus state was 48.3 MPa, with astandard deviation of 2.4 MPa.

The original hydrogel samples could also be doped using copper(I). Thesamples were doped by soaking overnight in a doping solution containingCuCl, NH₄OH, a complexing agent, and sodium meta-bisulfite, a reducingagent. The hydrogels that were doped with CuCl remained clear and paleyellow. Samples did not appear to shrink or swell as a result of doping.However, the samples oxidized readily in air to copper (II) as a resultof the presence of oxygen. Oxidation in air resulted in a colortransition from yellow to green, and then to the characteristic brightblue of a copper (II) hydrogel. The sample became harder, smaller, andsmall beads of water became evident on the outside of the sample.

An array of hydrogels were prepared to be 20, 25, 30, and 35%4-vinylpyridine by mass. Increasing the amount of ligand in thehydrogels resulted in a darker yellow color and an increase inbrittleness. These samples were then doped with decreasingconcentrations of CuCl₂ and a constant concentration of urea (0.025 M).After the samples were doped with CuCl₂, those containing greateramounts of ligand were a darker blue and stiffer than those with lessligand.

TABLE 2 The results of an indentation test for the first eight samplesdepicted in E represents the average Young's modulus, and σ representsthe standard deviation of the measurements. Sample a b c d e f g h[CuCl₂] 0.25M 0.25M 0.25M 0.25M 0.1M 0.1M 0.1M 0.1M % ligand 20 25 30 3520 25 30 35 E  3.9 MPa 931 kPa 2.66 MPa N/A  3.0 MPa  7.4 MPa 3.4 MPa1.1 MPa σ 120 kPa  83 kPa  160 kPa n/a 540 kPa 340 kPa  90 kPa  10 kPa

Copper(II)-doped samples could be converted to copper(I)electrochemically. To facilitate the experiment, the original hydrogelwas prepared on top of a glassy carbon electrode. The sample was dopedwithout removal from the electrode overnight in 0.5 M CuCl2 and 0.025 Murea. Electrochemical reduction of the sample was conducted in 15 mL ofan electrolyte that was also 0.5 M CuCl₂ and 0.025 M urea. The samplewas initially hard, glassy, and bright blue. The electrolyte solutionwas initially bright blue. After reduction overnight at −0.2 V, theelectrolyte turned a pale blue-green and large masses of copper metalwere found to have precipitated around the sample. The sample itselfbecame much softer and turned a clear, very light sea green dotted withelemental copper particles. By adjusting the composition of theelectrolyte, it is possible to completely suppress Cu(0) formiation.

A pressure dependent rate of reduction has been observed when the sampleis compressed during the electrochemical transformation. The iron-basedhydrogels also exhibit this behavior. FIG. 3. Copper(I)-based gels canalso be converted by electrochemical oxidation to Cu²⁺. The oxidizedsamples are blue in color and stiffer as expected.

Typical Hydrogel Preparation for Copper EPEHs:

Sodium (4-styrene sulfonate) (SS, 1.6 g, 7.77 mmol), 4-vinylpyridine(VP, 0.41 mL, 3.80 mmol), and poly(ethylene glycol) diacrylate, (PEG-DA,M_(n)=575, 100 μL, 0.194 mmol) were combined with 9 mL of deionizedwater and gently heated to below 40° C. until all solids were dissolved.The mixture was purged with N₂ for 1 min. Ammonium persulfate (APS, 60mg, 2.2 mol %) was added as radical initiator for copolymerization.Depending on the dimensions of the sample being prepared, 2 to 3 mL ofthe reaction mixture was pipetted into a mold. For electrochemicalexperiments the mold for the sample was created by temporarily affixinga square glass cell using poly(dimethylsiloxane) adhesive to a Teflonbase bearing a freshly polished glassy carbon electrode (GCE). Themold/sample combination was then heated at 85° C. for 1.5 h.

Doping of PSS/PVP Thin Samples with Cu²⁺:

The doping solution was prepared to be 0.5 M in CuCl₂ and 0.025 M inurea. Samples were doped with 5 mL copper solution for every 2 mLhydrogel solution. For a typical doping solution, CuCl₂ (0.34 g, 2.5mmol) and urea (0.008 g, 0.13 mmol) were combined with 5 mL deionizedwater and stirred until all solids were dissolved. The doping solutionwas poured directly over the sample. Both sample and solution werecovered and allowed to sit overnight.

Doping of PSS/PVP Thin Samples with Cu⁺:

The doping solution was prepared to be 0.1 M in CuCl, 0.5 M in NH₄OH,and 0.25 M in sodium meta-bisulfite. Samples were doped with 5 mL coppersolution for every 2 mL hydrogel solution. For a typical dopingsolution, CuCl (0.05 g, 0.51 mmol), sodium meta-bisulfite (Baker, 0.25g, 1.3 mmol), and NH₄OH (Baker, 0.14 mL, 2.4 mmol) were combined with 5mL deionized water that had been purged with N₂ for 5 min The mixturewas stirred until all solids were dissolved. The solution was purged foranother minute, and poured over the sample. Both sample and solutionwere covered and allowed to sit overnight under N₂.

It should be understood that while this invention has been describedherein in terms of specific embodiments set forth in detail, suchembodiments are presented by way of illustration of the generalprinciples of the invention, and the invention is not necessarilylimited thereto. Certain modifications and variations in any givenmaterial, process step or formula will be readily apparent to thoseskilled in the art without departing from the true spirit and scope ofthe present invention, and all such modifications and variations shouldbe considered within the scope of the claims that follow.

What is claimed is:
 1. A material having a first non-zero elasticmodulus capable of reversibly changing the first non-zero elasticmodulus to a second non-zero elastic modulus in response to a redoxreaction occurring in the material.
 2. The material of claim 1 whereinthe redox reaction is caused by an electric potential applied across thematerial or by exposing the material to an oxidant or reductant.
 3. Thematerial of claim 1 wherein the first non-zero elastic modulus or secondnon-zero elastic modulus is maintained by the material after abatementof the redox reaction.
 4. The material of claim 1 wherein the material,in-whole or in-part, may reversibly change from substantially the firstnon-zero elastic modulus to substantially the second non-zero elasticmodulus and vice versa upon successive redox reactions occurring in thematerial, in-whole or in-part.
 5. The material of claim 1 wherein thematerial has a minimum elastic modulus and a maximum elastic modulus,wherein each of the first non-zero elastic modulus and the secondnon-zero elastic modulus may consist of a value at the minimum elasticmodulus, the maximum elastic modulus or a value therebetween wherein thefirst non-zero elastic modulus is greater than the second non-zeroelastic modulus or the first non-zero elastic modulus is less than thesecond non-zero elastic modulus.
 6. The material of claim 1 wherein thematerial defines a first shape while the material has the first non-zeroelastic modulus which is less than the second non-zero elastic modulus,wherein the first shape may be deformed to define a second shape andwherein the second shape is maintained after application to the materialof a redox reaction and wherein the material attains the second non-zeroelastic modulus.
 7. The material of claim 1 wherein the material is acomponent of a composite structure.
 8. A material reversibly cyclablebetween a first non-zero elastic modulus and a second non-zero elasticmodulus comprising one or more metal ions and a polymer having bothcrosslinks that do not depend on metal binding and functional groupsthat have oxidation-state specific binding constants to the metal ions.9. The material of claim 8 wherein the metal ions consist of only oneelement.
 10. The material of claim 8 wherein the metal ions consist ofat least two different elements and the polymer has both crosslinks thatdo not depend on metal binding and functional groups that haveoxidation-state specific binding constants to each element.
 11. Thematerial of claim 8 wherein the polymer contains one or more functionalgroups selected from the group consisting of: carboxylate, carboxylicacid, amide, sulfide, thiol, alkoxide, alcohol, phosphine, amine, amide,halogen, sulfonic acid, sulfonate, phosphonate and phosphonic acid;wherein the one or more functional groups bind to the metal ions. 12.The material of claim 8 wherein the one or more metal ions are selectedfrom the group consisting of: iron (II/III), copper (I/II), cobalt(II/III), palladium (0/II), silver (I/II), manganese (II,III,IV), nickel(0,II), ruthenium (II/III), rhodium (I/II/III), platinum (0/II) and gold(0/I/II).
 13. The material of claim 8, wherein the polymer comprisesmonomer residues from monomers having the structure H₂C═CR¹R², where R¹and R² are independently selected from the group consisting of —H,(C1-C8)alkyl, phenyl, aryl, heteroaryl, pyridinyl, pyrrolyl, thiophenyl,—C(═O)OR³, —C(═O)NR³R⁴, —C(═NR³)OR⁴ and —CH═CR³R⁴, where R³ and R⁴ areindependently selected from the group consisting of —H, (C1-C8)alkyl,phenyl, aryl, heteroaryl, pyridinyl, pyrrolyl and thiophenyl.
 14. Thematerial of claim 8 wherein the polymer is a hydrogel and comprises atleast one monomer residue selected from the group consisting of:4-vinylpyridine, acrylate, styrene sulfonate and polyethylene glycoldiacrylate.
 15. The material of claim 12 wherein the one or more metalions is selected from the group consisting of copper (I/II) and iron(II/III).
 16. The material of claim 8 wherein the polymer furthercomprises a least one conductive additive selected from the groupconsisting of: carbon nanotubes, chemically modified carbon nanotubes,graphene, partially-reduced graphene oxide, chemically modifiedgraphene, conducting polymers and nanoparticles.
 17. A method ofproducing a material that is reversibly cyclable between a firstnon-zero elastic modulus and a second non-zero elastic modulus,comprising: preparing a polymer comprising both crosslinks that do notdepend on metal binding and functional groups capable of havingoxidation-state specific binding constants to a metal ion; and dopingthe polymer with a solution containing the metal ion.
 18. The method ofclaim 17, further comprising reversibly cycling between a first non-zeroelastic modulus and a second non-zero elastic modulus by altering theoxidation state of the metal ion by a redox reaction or a redoxstimulus.
 19. The method of claim 17 wherein the solution furthercomprises at least one ligand with functional groups capable of bindingto the metal ion, wherein the functional groups are selected from thegroup consisting of: carboxylate, carboxylic acid, amide, sulfide,thiol, alkoxide, alcohol, phosphine, amine, amide, halogen, sulfonicacid, sulfonate, phosphonate and.
 20. The method of claim 17 wherein thesolution comprises a copper salt and urea.
 21. The method of claim 17wherein the solution comprises an iron salt and citric acid.
 22. Themethod of claim 21 wherein the iron salt is FeCl₂ or FeCl₃.
 23. Themethod of claim 17 wherein the polymer is a cross-linked hydrogel madeby a first step of polymerizing an aqueous solution comprising sodiumacrylate, sodium sulfonate, and polyethylene glycol diacrylate withammonium persulfate.
 24. The method of claim 18 wherein the redoxstimulus is delivered by application of an electric potential across thematerial.